High performance microelectronic devices often use solder balls or solder bumps for electrical interconnection to other microelectronic devices. For example, a very large scale integration (VLSI) chip may be electrically connected to a circuit board or other next level packaging substrate using solder balls or solder bumps. This connection technology is also referred to as “Controlled Collapse Chip Connection-C4” or “flip-chip” technology, and will be referred to herein as solder bumps.
According to solder bump technology developed by IBM, solder bumps are formed by evaporation through openings in a shadow mask which is clamped to an integrated circuit wafer. For example, U.S. Pat. No. 5,234,149 entitled “Debondable Metallic Bonding Method” to Katz et al. discloses an electronic device with chip wiring terminals and metallization layers. The wiring terminals are typically essentially aluminum, and the metallization layers may include a titanium or chromium localized adhesive layer, a co-deposited localized chromium copper layer, a localized wettable copper layer, and a localized gold or tin capping layer. An evaporated localized lead-tin solder layer is located on the capping layer.
Solder bump technology based on an electroplating method has also been actively pursued. The electroplating method is particularly useful for larger substrates and smaller bumps. In this method, an “under bump metallurgy” (UBM) layer is deposited on a microelectronic substrate having contact pads thereon, typically by evaporation or sputtering. A continuous under bump metallurgy layer is typically provided on the pads and on the substrate between the pads to allow current flow during solder plating.
An example of an electroplating method with an under bump metallurgy layer is discussed in U.S. Pat. No. 5,162,257 entitled “Solder Bump Fabrication Method” to Yung and assigned to the assignee of the present application. In this patent, the under bump metallurgy layer includes a chromium layer adjacent the substrate and pads, a top copper layer which acts as a solderable metal, and a phased chromium/copper layer between the chromium and copper layers. The base of the solder bump is preserved by converting the under bump metallurgy layer between the solder bump and contact pad into an intermetallic of the solder and the solderable component of the under bump metallurgy layer.